Quicksort, orpartition-exchange sort, is asorting algorithmdeveloped byTony Hoarethat,on average, makes O(n log n) comparisons to sortnitems. In theworst case, it makes O(n2) comparisons, though this behavior is rare. Quicksort is often faster in practice than other O(n log n) algorithms.[1]Additionally, quicksort's sequential and localized memory references work well with acache. Quicksort is acomparison sortand, in efficient implementations, is not astable sort. Quicksort can be implemented with anin-place partitioning algorithm, so the entire sort can be done with only O(log n) additional space used by the stack during the recursion.

Quicksort is adivide and conquer algorithm. Quicksort first divides a largelistinto two smaller sub-lists: the low elements and the high elements. Quicksort can then recursively sort the sub-lists.

The steps are:

Pick an element, called apivot, from the list.

Reorder the list so that all elements with values less than the pivot come before the pivot, while all elements with values greater than the pivot come after it (equal values can go either way). After this partitioning, the pivot is in its final position. This is called thepartitionoperation.

Recursivelyapply the above steps to the sub-list of elements with smaller values and separately the sub-list of elements with greater values.

Thebase caseof the recursion are lists of size zero or one, which never need to be sorted.